TRI PARALLEL MIXER The problem of “metallic sound like a flanger”

I have a problem with setting the potentiometers in TRI PARALLEL MIXER. A simple, classic signal path with an effects loop.

Guitar -> INPUT amplifier (clean channel) -> SEND loop -> TRI PARALLEL MIXER -> RETURN loop -> Power amplifier -> SPEAKER

The EHX Oceans 11 reverb is looped to TRI PARALLEL MIXER.

Description of the “metallic sound like a robot’s voice or like a flanger” problem:

1.
When the FX LEVEL knob on the Reverb EHX Oceans 11 is set to 12 and the DRY VOL, RETURN, and SEND knobs on the TRI PARALLEL MIXER are also set to 12, the outputs are summed, making you hear a metallic sound that sounds like Flange. When the DISTORTION channel on the amplifier is turned on, the metallic sound is very audible. The problem appears when the Reverb EHX Oceans 11 is turned on and off.

2.
When the Reverb EHX Oceans 11 has the FX LEVEL knob set to 12 and the TRI PARALLEL MIXER DRY VOL knobs set to 9 (muted or fully muted), there is a problem with the metallic flanger sound.

3.
On the Reverb EHX Oceans 11, when the FX LEVEL knob is set to 12 and the TRI PARALLEL MIXER DRY VOL knobs to 12, RETURN and SEND also to 9 (lowered), the problem of metallic flanger sound disappears. But then Oceans 11 can be heard very softly.

4.
When the FX LEVEL knob on the Reverb EHX Oceans 11 is set to MAX, the problem of metallic flanger sound disappears.

5.
When I connect to TRI PARREL to another loop, such as EHXSuperego or DELAY, it is the same problem. Only reducing the VOL or FX of the LVL of the connected FX removes the problem of the metallic flanger sound. This is best heard when you turn on the built-in DISTORTION before TRI PARALLEL.

Is it normal, is something broken, or am I setting the potentiometers wrong?